In a photostimulable phosphor imaging system, as described in U.S. Pat. No. Re. 31,847 reissued Mar. 12, 1985 to Luckey, an image storage panel comprising a photostimulable phosphor is exposed to an image wise pattern of high energy short wavelength radiation, such as x-radiation, to record a latent image pattern in the image storage panel. The latent image is read out by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photostimulable phosphor releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. To produce a signal useful in electronic image processing, the image storage panel is scanned in a raster pattern by a beam of stimulating radiation produced for example by an infrared laser deflected by an oscillating or rotating scanning mirror, and the emitted radiation is sensed by a photodetector such as a photomultiplier tube, to produce the electronic image signal.
In the conventional photostimulable phosphor imaging systems, the image storage panel is turbid (i.e. scattering) to both stimulating and emitted wavelengths of light. In such a turbid storage panel system, the minimum pixel size, and hence the resolution that can be achieved, corresponds to the scattered area of the scanning beam within the storage panel itself. It has been proposed that the resolution of a photostimulable phosphor imaging system may be greatly improved by making the image storage panel transparent, thereby enabling a reduction in the effective pixel size, since the apparent size of the beam is not increased by scattering. See for example, the article entitled "Laser-Stimulable Transparent CsI: Na Film for a High Quality X-ray Image Sensor" by Kano et al, Applied Physics Letters 48(17), Apr.28, 1986.
Since the MTF (Modulation Transfer Function-a measure of the ability of the system to record details) of the transparent photostimulable phosphor imaging system is limited mainly by the effective size of the scanning beam of stimulating radiation, the x-ray absorption of the storage panel may be increased by making it thicker, without increasing the effective size of the scanning beam. In this way, the signal-to-noise ratio of the x-ray detector may be improved. In the conventional turbid storage panels, the thickness was limited by the spreading of the scanning beam in the panel.
Unfortunately, the transparent storage panel has a drawback, in that a large fraction of the emitted light is totally internally reflected within the transparent storage panel and is not collected by conventional light detectors. The only emitted light that escapes from the surface of the transparent storage panel is that which is emitted in the solid angle subtended by the light rays incident at less than a critical angle to the surface.
FIG. 4 illustrates this problem of recovering emitted light from a transparent storage panel. A scanning beams of stimulating radiation is directed onto one side of a transparent image storage panel 10 having an index of refraction n.sub.p &gt;1. The scanning beam stimulates the emission of light in all directions within the storage panel 10. Only the emitted light rays within a solid angle subtended by angle .theta..sub.c (illustrated by solid lines in FIG. 4) can escape from the surface of the transparent storage panel 10. The rest of the rays (represented by dashed lines in FIG. 4) are trapped within the phosphor sheet by total internal reflection. The critical angle .theta..sub.c for total internal reflection is defined as: ##EQU1## where n.sub.air is the index of refraction for air=1.
In terms of solid angle, the ratio .epsilon. (herein called escape efficiency) of the emitted light escaping from one side of the sheet to the total light emitted from the phosphor is expressed as: ##EQU2## The total emitted light escaping from both the top and bottom of the sheet is proportional to 2 .epsilon.. For a transparent storage panel having an index of refraction of 1.6, this means that only about 11% of the light escapes from the top of the panel, and another 11% from the bottom, the remainder of the light is trapped within the panel by total internal reflection.
Collectors may be located on both sides of the storage panel to collect a total of 22% of the emitted light. Alternatively, a mirror surface can be placed on one side of the storage panel and light collected from the other side of the storage panel. In this case, the collection efficiently .epsilon. is: ##EQU3## where R is the reflectance of the mirror surface.
One solution to this problem is to employ a photostimulable phosphor sheet that comprises a photostimulable phosphor dispersed in a polymeric binder. The polymeric binder is selected such that its index of refraction matches that of the phosphor at the stimulating wavelength, but does not match that of the phosphor at the emitted wavelength. Thus, the emitted wavelength is scattered and is not trapped by total internal reflection, while the benefits of the transparent phosphur sheet are achieved for the stimulating wavelength. See Canadian Patent No. 1,175,647 issued Oct. 9, 1984 to Deboer and Luckey. Although the solution is ideal for a phosphor-binder type photostimulable medium, it does not solve the problem for an isotropic photostimulable medium such as the fused phosphor described in the above-referenced Kano et al. article. Furthermore, even in a phosphor-binder type photostimulable medium, the desired indices of refraction are difficult to achieve in practice. A rapid change in the index of refraction of a material with changes in wavelength, which is necessary for the phosphor-binder system to be transparent to stimulating wavelength and turbid for the emitted wavelength of light, is generally associated with an absorption peak. Obviously, the presence of an absorption peak near the wavelengths of interest is to be avoided if maximum efficiency is to be achieved from the system.
It is the object of the present invention to provide a radiation image storage panel having a transparent stimulable phosphor with an improved escape efficiency for emitted light.